The present invention relates to the production of panels of composite material provided with local stiffening, by the draping/molding technique.
This technique, which consists in draping on a suitable tool called a mold, pre-impregnated fibers of resin, applying on these fibers a film or bladder, drawing a vacuum between the bladder and the fibers, then placing the assembly thus obtained in an autoclave to subject it to a polymerization cycle ensuring the curing of the resin, permits obtaining panels of shapes and dimensions that are quite precise, adapted for various usages.
The invention relates more particularly to shaped panels that can constitute the nacelle of aircraft motors.
Such a nacelle is generally constituted by several composite panels produced in two parts and then assembled together. The shape of the panels is computed by aerodynamic specialists so as to limit the drag adjacent the air foil. As a general rule, it is usually curved and of large size. During production of each panel, it is important to keep this form so as to preserve the shapes and dimensions.
So as to rigidify the shape and to guarantee the dimension of these panels, local stiffening is used.
Different techniques are used at present.
The panel can be stiffened by a continuous cellular structure, covering all the surface of the panel. This cellular structure is sandwiched between two layers of fibers or pre-impregnated plies.
In the case of a single curing, during production of such a panel, upon placing under vacuum, the plies of the layer of pre-impregnated fibers have the tendency to sink into the open cells of the cellular structure. Thus, before the polymerization temperature of the resin is reached, the consistency of these fabrics is soft. At the end of the cycle, the panel thus formed comprises hollows which modify the aerodynamic shape.
To overcome these problems, there exist two solutions:
using a cellular structure with small cells such that the plies of the layer of fibers cannot sink into them. The principal drawback is the weight of the panel produced, because the smaller the size of the cells, the greater the density of the structure,
producing the panel in several steps. First, the layer of pre-impregnated fibers is cured, because once hardened, the layer cannot sink into the cells of the cellular structure. Upon the second curing, there is added the cellular structure and the last layer. This process requires producing the panel in several steps requiring a fairly long production time, as well as a large number of different tools.
Moreover, this technique has the drawback of being difficult to repair by the user, once the panel becomes damaged. In case of shock, moisture may seep into the cellular structure, degrading the structural properties of said structure and requiring a very precise repair according to the very strict rules.
Another technique consists in using local stiffeners, in the form of strips of cellular structure or foam strips. The use of a cellular structure gives rise to the same drawbacks, although reduced, as in the preceding technique, namely: sinking of the plies of fibers, the weight of the panel, the difficulty of repair, the need for several polymerization cycles, and the risk of moisture uptake as a result of shock.
Foam has the advantage of avoiding the pre-impregnated plies of fibers sinking in, because it has no open cells. It nevertheless has a major drawback: the plies of pre-impregnated fibers sink during cooling of the panel because of the difference of coefficient of expansion between said fibers and the foam. This problem exists moreover also with cellular structure. Thus, the materials used to produce stiffening strips (foam, cellular structure, . . . ) have in general a coefficient of expansion that is greater than the pre-impregnated materials. In the autoclave, at the polymerization temperature, the plies of pre-impregnated material solidify whilst the material of the stiffening strip dilates. During cooling of the piece, said material retracts to return to its initial position (before heating) and carries with it the solidified composite material, causing sinking. As a result, slight hollows appear on the panel, which modifies the fluid flow and generates unacceptable aerodynamic drag. Moreover, these hollows weaken the panel by generating the onset of flexure. In this case, the foam strip or structure necessarily has a structural role to compensate this force. This requires strict repair rules.
It is possible to avoid the use of a cellular structure by replacing it with localized shaped stiffeners. These localized stiffeners can have different shapes (Z, U, I, . . . as a function of the place where they are positioned. They are first made of a composite material, by the draping/molding technique, on tools specific to their shape. There must accordingly be as many tools as shapes to be produced. The production of the panel takes place in two steps: curing of the layer of pre-impregnated fibers, then cementing of the localized stiffeners.
This cementing operation requires very precise tools, because there exist problems of adaptation between the stiffeners and the panels, namely problems of contraction at the points of contact. This technique generates risks of loosening of the stiffeners, in addition to multiple polymerization operations.
Thus, in the present economic context, in which the highest quality is sought at the best price, the preceding techniques although used in industry have not been entirely satisfactory.
The present invention seeks to overcome the drawbacks of the known techniques by providing a process for the production of panels of composite material with stiffening strips, by the technique of draping/molding/polymerization, guaranteeing the shape and dimensions of the panels after demolding particularly by avoiding any sinking of the fibers into the stiffening strips, whilst permitting production in what can be a very short time and without requiring too complicated a production cycle.
To this end, the invention has for its object a process for the production of a panel of composite material with stiffening strips, in which a mold having the shape and size of the panel to be produced is draped with at least a first ply of fibers pre-impregnated with resin, resin placed at the desired positions on said ply at least stiffening strip, the stiffening strip as well as at least a portion of the remaining surface of said first play is covered with at least one second ply of fibers pre-impregnated with resin, then a bladder is emplaced, covering the above elements; there is established a suitable vacuum in the space between the mold and the bladder, the assembly is then placed in an autoclave to be subjected to a pre-established polymerization cycle for the resin, at suitable pressures and temperatures for said resin, and finally, the panel is demolded, characterized in that prior to said emplacement of the stiffening strip and/or of the second ply, there is interposed at least one layer, film or the like of a non-adherent material between at least one portion of the surface of the stiffening strip and one and/or the other of said plies.
With such a process, the non-adherent material, interposed between the layer of pre-impregnated fibers and a stiffening strip of the type with a cellular structure, avoids sinking of the plies into the cells of the cellular structure. It moreover avoids, during curing, this stiffening strip becoming secured to the layer of composite material. Thus, during cooling, the material used for the stiffening strip can be withdrawn without disturbing the solidified layer of composite material. The surface of the panel is thus not modified, which keeps its aerodynamic shape.
Moreover, as the stiffening strip or strips can be totally enclosed within said non-adherent material, there is no longer adhesion between the strips and the composite material. The panel is thus unaffected by the expansion of the strips and hence to the deformations which they can cause.
Also, the stiffening strips no longer being working and thus having no more force to transmit, any low density material which can nevertheless withstand the temperatures and pressures of the polymerization cycle, can be used to produce said strips. They can thus be particularly of a cellular or foam structure.
It is also to be noted that if desired, the stiffening strips can be withdrawn after demolding the panel, because they do not adhere to the composite material.
The process described above in the framework of practice including a single cure, is suitable also for two-stage curing, namely a first curing of the superposed plies constituting the first layer of composite material to be placed on the bottom of the mold, so as to rigidify this first layer, then a second cure after emplacement of the stiffening strips, non-adherent layers, second layer of composite material, so as to rigidify the assembly, each cure being of course carried out after emplacement of a bladder, placed under vacuum, and then autoclaved.
Such a polymerization in two stages is possible because there is no longer risk of the plies sinking due to the different coefficients of expansion of the material used.
The non-adhered material is for example selected from the group consisting of Teflon(copyright) and silicone-base compounds.
The invention also has for its object panels with stiffening strips, obtained according to the process.